Objective: Dry eye disease (DED) is a common condition in which deficiencies in one or more components of the lacrimal functional unit cause drying, irritation, and blurry vision, often leading to reduced productivity and quality of life. Most DED treatments address symptoms rather than underlying causes. Treatment development has been stymied by the fact that currently accepted clinical signs are difficult to accurately measure and correlate poorly with symptoms. A new clinical and research approach is needed in order to improve the clinical care of this common condition. This study investigates tear film instability (TFI) as a unifying mechanism of DED by testing the hypothesis that a) TFI stimulates the visual system and ocular surface sensory apparatus and b) repeated stimulation leads to the DED symptoms of visual disturbance and ocular discomfort. Methods: To properly test this hypothesis requires a determination of the relationships between TFI, optics and vision, and corneal sensation. Three aims will be completed. Aim 1 will determine the signature optical and visual effects of TFI, including traditional wavefront aberrations and a novel analysis and modeling of optical scatter. Fluorescence imaging of the tear film will be calibrated by wavefront to account for the effects of quenching. In Aim 2, the signature sensory effects of TFI will be compared to known corneal stimuli (Belmonte and Cochet-Bonnet esthesiometers and hyperosmolarity drop testing) using visual analog scales and matching and multi-dimensional scaling to determine: a) whether osmolarity levels and/or ocular surface exposure create characteristic sensory effects, and b) whether and how repeated corneal stimulation by TFI leads to hyperalgesia or sensory adaptation. Mathematical modeling of TFI, accounting for lateral flow and epithelial permeability, will be used to create hyperosmolarity maps which will serve as specific TFI stimuli for this aim. Within the same experimental framework and using the same subject pool, the results from Aims 1 and 2 will be combined to, for the first time, investigate the temporal association between optical, visual and sensory responses during TFI (Aim 3). According our model, the optical, visual, and sensory signatures observed during TFI will arise from corresponding changes in tear film thickness and surface exposure. Participants: Using the standard tear break up time test, subjects with a wide range of TFI but good visual acuity and no evidence of significant corneal or ocular surface damage will be selected. This relatively healthy sample permits investigation of the inputs leading to classic symptoms of dry eye disease. Relevance: TFI can be thought of as a unifying mechanism in DED in that, unlike the symptoms and endpoints that have been the focus of much previous DED research, it is present regardless of which aspect of the lacrimal functional unit is affected. A full and detailed understanding of how TFI inter-relates with the visual and neural systems to protect the eye from damage will be instrumental in the development of diagnostic and interventional strategies targeting the origin of DED symptoms (not just palliative treatments). !